linux-kernel - Re: [PATCH 1/1] nvdimm: allow exposing RAM carveouts as NVDIMM DIMM devices

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Message-ID: <68d3465541f82_105201005@dwillia2-mobl4.notmuch>
Date: Tue, 23 Sep 2025 18:16:05 -0700
From: <dan.j.williams@...el.com>
To: Michał Cłapiński <mclapinski@...gle.com>, "Mike
 Rapoport" <rppt@...nel.org>
CC: Ira Weiny <ira.weiny@...el.com>, Dan Williams <dan.j.williams@...el.com>,
	Dave Jiang <dave.jiang@...el.com>, Vishal Verma <vishal.l.verma@...el.com>,
	<jane.chu@...cle.com>, Pasha Tatashin <pasha.tatashin@...een.com>, "Tyler
 Hicks" <code@...icks.com>, <linux-kernel@...r.kernel.org>,
	<nvdimm@...ts.linux.dev>
Subject: Re: [PATCH 1/1] nvdimm: allow exposing RAM carveouts as NVDIMM DIMM
 devices

Michał Cłapiński wrote:
> On Fri, Aug 29, 2025 at 9:57 AM Mike Rapoport <rppt@...nel.org> wrote:
> >
> > Hi Ira,
> >
> > On Thu, Aug 28, 2025 at 07:47:31PM -0500, Ira Weiny wrote:
> > > + Michal
> > >
> > > Mike Rapoport wrote:
> > > > From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>
> > > >
> > > > There are use cases, for example virtual machine hosts, that create
> > > > "persistent" memory regions using memmap= option on x86 or dummy
> > > > pmem-region device tree nodes on DT based systems.
> > > >
> > > > Both these options are inflexible because they create static regions and
> > > > the layout of the "persistent" memory cannot be adjusted without reboot
> > > > and sometimes they even require firmware update.
> > > >
> > > > Add a ramdax driver that allows creation of DIMM devices on top of
> > > > E820_TYPE_PRAM regions and devicetree pmem-region nodes.
> > >
> > > While I recognize this driver and the e820 driver are mutually
> > > exclusive[1][2].  I do wonder if the use cases are the same?
> >
> > They are mutually exclusive in the sense that they cannot be loaded
> > together so I had this in Kconfig in RFC posting
> >
> > config RAMDAX
> >         tristate "Support persistent memory interfaces on RAM carveouts"
> >         depends on OF || (X86 && X86_PMEM_LEGACY=n)
> >
> > (somehow my rebase lost Makefile and Kconfig changes :( )
> >
> > As Pasha said in the other thread [1] the use-cases are different. My goal
> > is to achieve flexibility in managing carved out "PMEM" regions and
> > Michal's patches aim to optimize boot time by autoconfiguring multiple PMEM
> > regions in the kernel without upcalls to ndctl.
> >
> > > From a high level I don't like the idea of adding kernel parameters.  So
> > > if this could solve Michal's problem I'm inclined to go this direction.
> >
> > I think it could help with optimizing the reboot times. On the first boot
> > the PMEM is partitioned using ndctl and then the partitioning remains there
> > so that on subsequent reboots kernel recreates dax devices without upcalls
> > to userspace.
> 
> Using this patch, if I want to divide 500GB of memory into 1GB chunks,
> the last 128kB of every chunk would be taken by the label, right?
> 
> My patch disables labels, so we can divide the memory into 1GB chunks
> without any losses and they all remain aligned to the 1GB boundary. I
> think this is necessary for vmemmap dax optimization.

As Mike says you would lose 128K at the end, but that indeed becomes
losing that 1GB given alignment constraints.

However, I think that could be solved by just separately vmalloc'ing the
label space for this. Then instead of kernel parameters to sub-divide a
region, you just have an initramfs script to do the same.

Does that meet your needs?